Lots of tests rely on llvm-lto being present, but LLVM_ENABLE_PIC=OFF currently disables building that executable.
There's no reason for not building llvm-lto with LLVM_ENABLE_PIC=OFF so just build it. r191042 moved it into a "if (!WIN)" block at the time, and then 211852 made that "if(NOT CYGWIN AND LLVM_ENABLE_PIC)" -- but that's only needed for LTO (the ld64 plugin), not for the llvm-lto binary.